The viral DNA of Moloney leukemia and Harvey sarcoma viruses is being mapped using restriction endonucleases. The regions of the sarcoma virus genome which are of mouse leukemia and of rat origin will be determined. The specificity of the site of integration by leukemia virus will be studied by endonuclease cleavage of cellular DNA's carrying integrated viral genomes. BIBLIOGRAPHIC REFERENCES: Gianni, A.M., Hutton, J.R., Smotkin, D. and Weinberg, R.A.: Proviral DNA of Moloney Leukemia Virus: Purification and Visualization. Science 191: 569, 1976. Smotkin, D., Yoshimura, F. and Weinberg, R.A.: Infectious Linear Unintegrated DNA of Moloney Murine Leukemia Virus. J. Virol. 20: 621-626, 1976.